Shimming device for superconducting magnet

Abstract

A shimming device sequentially records a history of conducted shim setting, momentarily dynamically calculates compensation values corresponding to the record by the aid of a characteristic function, and dynamically controls the shim coil by the aid of the compensation values thus obtained, to thereby compensate magnetic field fluctuation of a superconducting magnet which is attributable to the setting of the shim, and stabilizes the magnetic field in an NMR superconducting magnet. This device includes a shim setting recorder ( 230 ) that records a set value of the shim, a type of set operation, the operation amount of the setting, and a time at which the setting is conducted, in association with each other for each setting; generating compensation function supply ( 210 ) that supplies a generating compensation function that is a function having the type of the operation, the operation amount, and the amount of time as factors, and is set so as to approximately compensate a magnetic field fluctuation attributable to individual setting; compensation value computer ( 240 ) that refers to the generating compensation function supply ( 210 ) and the shim setting recorder ( 230 ), computes a compensation value at the time t by the aid of a compensation function that is configured by, and outputs the computation result; a shim coil driver ( 300 ) that drives the shim coil ( 400 ) by the aid of a sum of the compensation value obtained by the compensation value computer ( 240 ), and a latest set value.

The invention claimed is: 
     
       1. A shim coil control device that compensates stability and homogeneity of a magnetic field of a superconducting magnet with a shim coil,
 wherein the shin coil control device comprises a computer programmed to output a signal to a shim coil driving device for controlling at least one term of the shim, and 
 wherein the shim coil control device is programmed to provide 
 (1) a shin setting recorder that records, when the setting of the shim is conducted:
 a set value u of the shim; 
 a type w of the set operation; 
 an amount s of the operation of the setting; and 
 the time t of setting, 
 
 
       by associating each setting with an index n as (w n , u n , s n , t n ), wherein the index n is a value of 0 to N−1, where the number of times of setting is N, and the value of index n associated with the setting finally conducted is N−1;
 (2) generating compensation function supply that supplies a preset generating compensation function f w (s,t′) that is a function having the type w of the operation, the amount s of the operation of the setting, and the amount of time t′ as arguments, the generating compensation function having a property to become 0 when the amount of time t′ is infinite, and is set so as to approximately compensate a magnetic field fluctuation attributable to individual setting; 
 (3) compensation value calculator that refers to the generating compensation function supply and the shim setting recorder, and calculates a compensation value at the time t by the aid of a compensation function g(t) that is a function of the time t which is expressed as g(t)=Σf wn (s n , t−t n ) by the generating compensation function and the setting of the shim conducted before the time t; and 
 (4) an output value computer that computes a compensation value u′ by adding the compensation value obtained by the compensation value calculator and a latest set value u N-1  that is u′=u N-1 +g(t). 
 
     
     
       2. The shim coil control device according to  claim 1 , wherein the generating compensation function is expressed as f w   s=s0 (t′)×h(s/s 0 ) assuming that the generating compensation function f w (s 0 , t′) when the operation amount s of setting has a specific value s 0  is a species compensation function f w   s=s0 (t′) which is a function of the amount of time t′, the operation amount s of setting is normalized by the aid of s 0 , and a polynomial function h of the normalized operation amount s/s 0  is h(s/s 0 ).